This invention relates to a selective call radio receiving method and a selective call radio receiving device for receiving a radio communication signal by using a predetermined number of threshold levels derived from the radio communication signal.
For reception by such a selective call radio receiving device, the radio communication signal is propagated from a base station and includes a synchronization signal periodically and carries a communication datum. The radio communication signal may be an analog signal.
In the manner which will later be described in greater detail, a conventional selective call radio receiving device of this type comprises a receiving arrangement for receiving the radio communication signal to produce a voltage signal. Connected to the receiving arrangement, a detecting arrangement detects in the voltage signal a predetermined number, such as three, of detected threshold levels by discriminating between signal levels of a particular pattern included in the synchronization signal and a data signal which represents the communication datum as a sequence of received codes of a multiplicity, such as 256, of received symbol levels. Responsive to the detected threshold levels, a judging arrangement judges the data signal to produce a multilevel signal, such as a four-level signal. The number of levels of the multilevel signal is equal to one plus the predetermined number. In this manner, the selective call receiving device can reproduce the communication datum carried by the radio communication signal.
The detecting arrangement is inplemented in practice by a discriminator or an A/D converter. Inasmuch as such a signal conversion unit is used in converting the voltage signal primarily to a binary signal in various selective call radio communication devices, selection of parts of the detecting arrangement has been restricted to conversion of the voltage signal to the binary signal. This restriction is unavoidable even if the selection is for a four-level particular pattern for use in calculating three detected threshold levels. Moreover, the detecting arrangement has a conversion characteristic which is inevitably subject to a deviation and, through aging, to a shift. As a result, reproduction of the communication datum has been incorrect.
In order to cope with the shift, a countermeasure has been proposed to calculate a mean value of whole levels of the four-level particular pattern being received and a difference between the mean value and a middle one of the three detected threshold levels. The difference is used in commonly shifting the three detected threshold levels to provide three corrected threshold levels for use in judging the received codes of the communication datum. The proposal is, however, incapable of coping with the deviation.
In addition, the radio communication signal may be distorted either at instants of its propagation from the base station or while transmitted from the base station to the selective call radio receiving device. This inherent distortion of the radio communication signal makes it impossible to correct the detected threshold levels into the corrected threshold levels and results also in a reduced correctness of the reproduction.